This invention relates to a hydraulic brake system for automotive vehicles which is provided with a master cylinder and a hydraulic power booster connected upstream of the master cylinder and having a pressure chamber wherein an auxiliary pressure depending on the actuating force is established by means of a pedal-operable brake valve. Two brake circuits of the automotive vehicle are pressurizable by the braking pressure generator which consists of the master cylinder and of the power booster. In the brake system, first valve means are provided by means of which, in dependence on the operating signals of slip control electronics, the pressure supply to the brake circuits is interrupted and pressure medium is tapped from the brake circuits. Second valve means are provided which in case of control can be switched over and establish a hydraulic communication between two wheel brakes of one vehicle axle. The wheel brakes of one brake circuit are arranged diagonally opposite at the vehicle and the wheel brakes are connectible with the pressure chamber of the hydraulic power booster.
A hydraulic brake system featuring the above characteristics is described in the German patent application No. P 33 20 822.0. The brake system described in the application has a hydraulic power booster with a pressure chamber wherein an auxiliary hydraulic pressure may be established in dependence on an actuating force exerted on a brake pedal. To this end, a lever mechanism is fixed to a push rod connected with the brake pedal by means of which a brake valve can be operated. The brake valve is designed so as to ensure that in the release position of the brake a hydraulic communication exits between the pressure chamber of the hydraulic power booster and an unpressurized supply reservoir. When a force is applied to the brake pedal or when there is a displacement of the push rod connected with the brake pedal, a piston of the brake valve moves into an operating position so that the pressure of a pressure accumulator may enter the pressure chamber of the hydraulic power booster. This ensures that a booster piston is displaced in the direction of actuation. The result of a displacement of the booster piston of the hydraulic power booster is that the pistons of a master cylinder connected downstream of the hydraulic power booster are displaced, and the working chambers of the master cylinder are pressurized.
In the brake system described in the noted German patent application, the master cylinder has a piston and thus limits a working chamber where a first brake circuit is connected. The first brake circuit has two wheel brakes of an automotive vehicle, with the wheel brakes being arranged diagonally opposite at an automotive vehicle. A second brake diagonal or a second brake circuit hydraulically communicated with the pressure chamber of the hydraulic power booster. In the known brake system, upstream of each wheel brake of the front axle of the automotive vehicles, a 2/2-way valve is connected which may be electrically switched over, which normally adopts an open position, and which may be switched into a locking position by slip control electronics. Further, a 2/2-way valve is connected to each wheel brake of the front axle of the automotive vehicles, which likewise is operable by slip control electronics and which upon actuation by the slip control electronics establishes a hydraulic communication between the respective wheel brake and an unpressurized supply reservoir. In the brake diagonal connected to the pressure chamber of the hydraulic power booster, a normally open solenoid valve is arranged at the respective wheel brake of the rear axle of the automotive vehicle. The valve is likewise controllable by the slip control electronics. Moreover, a normally closed solenoid valve is connected at this wheel brake by means of which the braking pressure of the respective rear axle of the automotive vehicles can be reduced.
A further component of the known brake system is a change-over valve which at the beginning of the brake slip control may be switched into an open position and which thus establishes a hydraulic communication between the pressure chamber of the hydraulic power booster and the working chamber of the master cylinder. As soon as such a communication is established, a hydraulically operable valve arrangement is switched over, and the wheel brakes arranged at the rear axle of the automotive vehicle hydraulically are connected with each other.
In case of control the pressure may be adjusted individually in the wheel brakes of the front axle whereas the braking pressure in the wheel brakes of the rear axle of the automotive vehicle may only be controlled together. The advantage of this arrangement is that two electromagnetically operable hydraulic valves can be eliminated at the rear axle of the automotive vehicle.
A disadvantage of the brake system described above is that in case of a circuit failure at the rear axle of the automotive vehicle, no braking effect will result at either wheel brake of the real axle. It is thus an object of this invention to further develop the described brake system so as to ensure that in case of a circuit failure the disturbed part of the brake system is switched off in a predetermined manner.